It depends of the games you play.
Most of the complains are related to Far Cry 5.
I have encountered the same issue, the frame rates are all over the map.
It could be related to the 6 cores / 12 threads, since games work best on 4 cores / 8 threads.
So if you mainly play Far Cry 5 (Ubisoft engine), then stay away from the i5 9600k, otherwise is a very good CPU for gaming.
I just bought it for 4K video editing and haven't encountered any issues thus far. Then again, editing 4K content in Adobe is a bit different than playing an intensive game like Far Cry, GTA, or even CSGO or Apex. This thing cuts like butter, though, let me tell you. And that's without creating Proxies...